Electronic displays can use transmissive or emissive materials to generate pictures or light. Emissive materials are usually phosphorescent or electroluminescent materials. Examples are inorganic electroluminescent materials such as applied in thin film and thick film electroluminescent displays (EL-displays, for example thin film TFEL displays as manufactured by Sharp, Planar, LiteArray or iFire/Westaim). Another group is organic electroluminescent materials (such as Organic Light Emitting Diode OLED material) deposited in layers comprising small molecule or polymer technology or phosphorescent OLED, where the electroluminescent materials are doped with a phosphorescent material. Yet another group of materials are phosphors, commonly used in the well-established cathode ray tubes (CRT) or plasma displays (PDP) and even in emerging technologies like laser diode projection displays where the laser beam is used to excite a phosphor imbedded in a projection screen.
Two basic types of displays exist: fixed format displays which comprise a matrix or array of “cells” or “pixels” each producing or controlling light over a small area, and displays without such a fixed format, e.g. a CRT display. For fixed format, there is a relationship between a pixel of an image to be displayed and a cell of the display. Usually this is a one-to-one relationship. Each cell may be addressed and driven separately. Emissive, fixed format especially direct view displays such as Light Emitting Diode (LED), Field-Emission (FED), Plasma, EL and OLED displays have been used in situations where conventional CRT displays are too bulky and/or heavy and provide an alternative to non-emissive displays such as Liquid Crystal displays (LCD). Fixed format means that the displays comprise an array of light emitting cells or pixel structures that are individually addressable rather than using a scanning electron beam as in a CRT. Fixed format relates to pixelation of the display as well as to the fact that individual parts of the image signal are assigned to specific pixels in the display. Even in a color CRT, the phosphor triads of the screen do not represent pixels; there is neither a requirement nor a mechanism provided, to ensure that the samples in the image in any way align with these. The term “fixed format” is not related to whether the display is extendable, e.g. via tiling, to larger arrays. Fixed format displays may include assemblies of pixel arrays, e.g. they may be tiled displays and may comprise modules made up of tiled arrays which are themselves tiled into supermodules. Thus “fixed format” does not relate to the fixed size of the array but to the fact that the display has a set of addressable pixels in an array or in groups of arrays. Making very large fixed format displays as single units manufactured on a single substrate is difficult. To solve this problem, several display units or “tiles” may be located adjacent to each other to form a larger display, i.e. multiple display element arrays are physically arranged side-by-side so that they can be viewed as a single image. Transferring image data by packetized data transmission to the various display devices makes segregation of the displayed image into tiles relatively easy. At the junction of the tiles, usually some means to hide the join is applied. Such could be an opaque mask, as is for instance done in the case of tiled LCD displays, where the image of individual LCD panels is projected on a black matrix. To maintain a uniform appearance to the display, this mask is extended over the complete surface of the display and comprises an array of openings that coincide with the light emitting pixel structures of the display, or an array of openings that coincides with a group of light emitting pixel structures of the display (e.g. array of 4×4 pixels in one opening of the mask). OLED displays provide certain advantages for tiled displays such as light-weight, ease of manufacture, wide angle of view, and the ability to use back-connectors which allows close tiling with the smallest joint between tiles.
When making color displays, the colors are obtained through mixing light from primary colors such as but not limited to red, green and blue. For fixed format, emissive displays separate or stacked individual “primary” emitter layers generate these colors. If the primary emitter layers are applied next to each other and usually close to each other, then from a certain minimum distance onwards (compounding distance), the observer is not able to distinguish the primary emitters but sees only one resulting color. Most color displays are bi-color or full color referring to respectively two primaries or at least three primary emitters per pixel.
In order to be able to generate as many colors possible, including white, at least three primary emitters are required with the emitted wavelengths of each as close as possible to pure colors such as pure red, pure green and pure blue, for example. The theory of color perception is well known, for example from the book “Display Interfaces”, R. L. Myers, Wiley, 2002. Primaries exist as mathematical constructs only and then lie outside the range of real-world colors. A more useful color space and color co-ordinate system has been standardized, e.g. the CIE chromaticity diagram. Typically in fixed format displays red, green and blue pixel elements are used, typically called RGB pixel elements. For ease of production, the area sizes of the three color emitters, i.e. the RGB pixel elements, are made equal. A CIE chromaticity diagram with the locations thereon of typical OLED and LED materials is shown in FIG. 1. The locations on this diagram are shown for a typical OLED display: red, RO; green, GO and blue, BO as well as for an LED display: red, RL; green, GL; blue, BL.
Materials used to form a pixel element have a certain light output, which increases with increased delivered electrical charge per time unit and per area unit. This is sometimes called an L–J characteristic, light output (Nit or cd/m2) versus current density (mA/cm2). FIG. 2 shows such a curve for a green OLED material. Each unit of charge is trapped for a short time and then released with reduced energy, the difference being emitted as photon(s). This characteristic is unique for the relevant material. The characteristic can be described by an analytical function quite accurately—see FIG. 2. Another characteristic is the change of light output over time at a given current density. Some materials age with the accumulated amount of trapped charge over time. In addition, this is also a unique characteristic or property of the material (See FIG. 14).
Finally, emitters for fixed format displays have a certain emissive spectrum. Each material has a different dominant wavelength as well. This determines unambiguously what colors can be generated with a pixel.
LG electronics, Korea has disclosed (SID 2002, International Symposium Digest of Technical Papers, p. 1178–1181) a display panel with a 4-emitter pixel, of which two are red. The article clearly states that this is for compensating brightness (luminous efficiency), not lifetime and the areas of all four emitters are the same.
Similar ‘superpixels’ can be found in the LED display industry. However, in this case the target is to merely match the luminous efficiency and not the lifetime, as in all circumstance the LED outlives other components such as power supplies or cooling fans.
The usual trend in the industry is to use equal size emitters for displays and to develop improvements of the weakest material. The reason for this is optimization of various spin coating, shadow masking or other deposition processes. This inherently leads to non-optimized use of materials in the display.
In particular, OLED's are used for small handheld devices such as mobile or cellular phones, smart phones, super phones, PDA's, etc. These devices need to be read in a variety of incident light intensities. However, the battery power consumption needs to be kept with certain limits over the complete lifetime of the product in order to avoid user dissatisfaction.